1. Field of the Invention
The present invention relates to a gallium nitride group compound semiconductor light-emitting device, which is capable of emitting light from the blue region to the ultraviolet region of the spectrum, such as a light-emitting diode and a semiconductor laser; and a method for producing the same.
2. Description of the Related Art
FIG. 19 is a cross-sectional view showing the structure of the gallium nitride group compound semiconductor laser disclosed in Japanese Laid-Open Publication No. 8-97507. The gallium nitride group compound semiconductor laser is fabricated by a metal organic chemical vapor deposition method (an MOCVD method). Hereinafter, the structure of the gallium nitride group compound semiconductor laser and a method for fabricating the same will be described.
First of all, a sapphire substrate 101 is inserted into an MOCVD apparatus. Then, an N-type GaN buffer layer 102, an N-type AlGaN lower cladding layer 103, an InGaN active layer 104, a P-type AlGaN upper cladding layer 105 and an N-type AlGaN internal current constricting layer 107 are sequentially grown on the sapphire substrate 101 in this order.
Next, the sapphire substrate 101 having the above-described layers provided thereon, namely a wafer, is taken out from the MOCVD apparatus. Then, the N-type AlGaN internal current constricting layer 107 is etched by photolithography to form a stripe-shaped opening. As a result, a current constricting layer with an opening is formed.
Thereafter, the above-described wafer is again inserted into the MOCVD apparatus. Then, a regrowth step is performed, and a P-type AlGaN upper cladding layer 108 and a P-type GaN contact layer 109 are sequentially formed in this order on the N-type AlGaN internal current constricting layer 107.
Finally, a P-side electrode 110 and an N-side electrode 111 are formed. In the above-described manner, the gallium nitride group compound semiconductor laser having the structure shown in FIG. 19 is completed.
According to this type of gallium nitride group compound semiconductor laser, in the case where the internal current constricting layer 107 is etched using wet etching or dry etching to form a stripe-shaped opening until a surface of the P-type AlGaN upper cladding layer 105 is exposed and then the P-type AlGaN upper cladding layer 108 is regrown in the MOCVD apparatus so as to cover the exposed surface of the P-type AlGaN upper cladding layer 105 and surfaces of the N-type AlGaN internal current constricting layer 107, the substrate temperature needs to be raised to about 1050.degree. C. (A substrate temperature means a temperature of a substrate having layers provided thereon.)
As a result, during a rise in the substrate temperature, an increase in surface roughness at the exposed surface of the P-type AlGaN upper cladding layer 105; a change in the width of the striped-shaped opening; and a deformation of the stripe-shaped opening formed in the N-type AlGaN internal current constricting layer 107 are generated. Consequently, electrical characteristics of the gallium nitride group compound semiconductor laser are deteriorated due to high resistance at the regrowth interface, and optical characteristics are deteriorated due to the change in the width of the stripe-shaped opening and the deformation of the stripe-shaped opening. Thus, the above-described gallium nitride group compound semiconductor laser has problems of reduced device characteristics.
The above-described problems will be described in detail with reference to FIG. 20. Since the step of regrowing the regrowth P-type AlGaN upper cladding layer 108 on the exposed surface of the AlGaN upper cladding layer 105 (the surface is exposed by etching) is performed at a high temperature of about 1050.degree. C., as shown in FIG. 20, a P-type impurity escapes by evaporation in the gas phase from the exposed surface of the P-type AlGaN upper cladding layer 105. As a result, a defect is caused on the exposed surface of the P-type AlGaN upper cladding layer 105 and surface roughness is increased at the exposed surface of the upper cladding layer 105. Accompanying this phenomenon, the stripe-shaped opening in the current constricting layer 107 is deformed. Moreover, Si (an N-type impurity) evaporates in the gas phase from the current constricting layer 107. As a result, surface roughness is increased at the surface of the current constricting layer 107. Consequently, crystallinity of the regrowth P-type AlGaN upper cladding layer 108, which is grown on the current constricting layer 107, is deteriorated. Thus, the surface condition of the regrowth P-type AlGaN upper cladding layer 108 is deteriorated.
When impurities escape from the interface between layers, resistance becomes high at the interface, thereby deteriorating the electrical characteristics. More specifically, forward voltage, operating voltage, and threshold voltage are increased. As a result, a light emitting pattern cannot be stabilized.
According to the above-described gallium nitride group compound semiconductor laser, the optical characteristics are deteriorated due to a change in the width of the stripe-shaped opening and deformation of the stripe-shaped opening. As a result, crystallinity of the regrowth P-type AlGaN upper cladding layer 108 is deteriorated, thereby causing the deteriorated surface condition of the regrowth upper cladding layer 108. Thus, reliability of the semiconductor laser is reduced.
According to the above-described conventional gallium nitride group compound semiconductor laser, the regrowth P-type AlGaN upper cladding layer 108 is grown directly on the exposed surface of the P-type AlGaN upper cladding layer 105 and the surfaces of the current constricting layer 107 at a high temperature. Therefore, due to damage caused by heat, the electrical and optical characteristics of the semiconductor laser are deteriorated, thereby reducing its reliability.
At present, however, no suitable wet etching solution for etching a gallium nitride group compound semiconductor is known. Therefore, in the step of removing the growth layer of the gallium nitride group compound semiconductor, it is difficult to leave the layer so as to have a required thickness, and good reproducibility, and to expose the surface of the desired growth layer by using a wet etching solution.
Also, when an internal current constricting layer is etched by photolithography in an ambient atmosphere to form a stripe-shaped opening and thus forming a current constricting layer with an opening, an impurity such as C or O attaches to the exposed surface of the upper cladding layer 105. As a result, when the regrowth upper cladding layer 108 is grown on the exposed surface of the upper cladding layer 105, interface level is generated at the regrowth interface. Consequently, series resistance at the interface and forward voltage increase. Thus, electrical characteristics are further deteriorated.
A gallium nitride group compound light-emitting diode having a current blocking layer instead of the current constricting layer has the same problems as those described above.